Calcium-magnesium-aluminum-silicate (CMAS) infiltration is a phenomenon that is linked to thermal barrier coating (TBC) spallation in hot section turbine components.
Thermal barrier coatings are utilized on hot section engine components including combustor section and turbine section components to protect the underlying base materials from high temperatures as a result of the flow of hot gases of combustion through the turbine. These hot gases of combustion can be above the melting point of the base materials, which typically are superalloy materials, being based on iron, nickel, cobalt and combinations thereof. The thermal barrier coatings provide passive protection from overheating, and are used in conjunction with cooling airflow that provides active cooling protection.
Under service conditions, these thermal barrier-coated hot section engine components can be susceptible to various modes of damage, including erosion, oxidation and corrosion from exposure to the gaseous products of combustion, foreign object damage and attack from environmental contaminants. Environmental contaminants that can be present in the air include sand, dirt, volcanic ash, sulfur in the form of sulfur dioxide, fly ash, particles of cement, runway dust, and other pollutants that may be expelled into the atmosphere, such as metallic particulates, such as magnesium, calcium, aluminum, silicon, chromium, nickel, iron, barium, titanium, alkali metals and compounds thereof, including oxides, carbonates, phosphates, salts and mixtures thereof. These environmental contaminants are in addition to the corrosive and oxidative contaminants that result from the combustion of fuel. These contaminants can adhere to the surfaces of the hot section components, which are typically thermal barrier coated.
At the operating temperature of the engine, these contaminants can form contaminant compositions on the thermal barrier coatings. These contaminant compositions typically include calcia, magnesia, alumina, silica (CMAS), and their deposits are referred to as CMAS. At temperatures above about 2240° F., these CMAS compositions may become liquid and infiltrate into the TBC. This infiltration by the liquid CMAS destroys the compliance of the TBC, leading to premature spallation of the TBC. In addition to the compliant loss, deleterious chemical reactions with yttria and zirconia within the TBC, as well as with the thermally grown oxide at the bond coating/TBC interface, occur and result in a degradation of the coating system.
The spallation due to CMAS infiltration has become a greater problem in jet engines as their operating temperatures have increased to improve efficiency, as well as in engines operating in the Middle East and in coastal regions. High concentrations of fine sand and dust in the ambient air can accelerate CMAS degradation. A typical composition of CMAS is, for example, 35 mole % CaO, 10 mol % MgO, 7 mol % Al2O3, 48 mol % SiO2, 3 mol % Fe2O3 and 1.5 mol % NiO. And of course, spallation of the TBC due to exposure to CMAS at elevated temperature only sets the stage for more serious problems. Continued operation of the engine once the passive thermal barrier protection has been lost leads to rapid oxidation of the base metal superalloy protective coating and the ultimate failure of the component by burn through or cracking. In fact, such significant distress has been observed in both military and commercial engines.
Various solutions to the problem of CMAS degradation have been attempted. However, as operating temperatures of engines have gradually trended higher, ever more effective treatments are required. What is needed is a TBC system that is resistant to CMAS penetration at elevated temperatures.